The invention relates to refractory cements. In particular, the invention is concerned with powdered or granular refractory compositions suitable for use as ramming, vibratory or gunning cements.
The basic metal fabrication and processing industry is very dependent on the type and quality of refractory linings available for the furnacing equipment utilized by that industry. Whether it's steel production employing one of the variations of the oxygen steel making processes, cupolas used in foundries, or induction furnaces, refractory inner linings are critical. These linings, at a minimum, must be stable at the high temperatures employed, physically strong enough to resist erosion from the turbulence in the melt, and chemically resistant so as not to be decomposed by the highly reactive mix of the melt. The inner linings can be constructed of mortared refractory brick but obviously the preferred construction would be a monolithic liner and one that can be patched or repaired in place without a complete shut-down of the furnace, or at least without any prolonged shut-down.
The prior art is replete with cement compositions, which fall generally into either wet or dry consistencies depending on how the cement is to be used. Cements are also separable into two additional categories viz. heat bonded and chemcially bonded. The selection of wet or dry is based primarily on how the cement is to be applied or utilized. If the cement is to be used as a vibratory cement, then the powdered cement mixture is dry. If on the other hand the mix is to be used wet such as in wet ramming or gunning operations, the dry cement ingredients are admixed with a predetermined quantity of liquid, usually water, to give the desired consistency. With respect to chemically bonded (cold set) versus heat bonded cements, the latter is generally selected where the physical conditions of the furnacing are such that the cement mix can be put in place and fired at a temperature in excess of 1000.degree. C; these result in highly refractory monolithic structures. In some cases, however, cold setting cements such as Portland or gypsum cements, soluble alkali silicate or phosphate bonded cements, or sulfate or chloride bonded cements are used; these develop a substantial amount of strength at room temperature.
A low temperature setting phosphate or phoshoric acid cement is described by Hansen et al in U.S. Pat. No. 2,852,401 which is suitable for wet cement applications. The cement is made up of a refractory aggregate that is inert in the mixture at room temperature, a minor amount of an aluminous material which is reactive with phosphoric acid at room temperature, and phosphoric acid e.g. a mixture of 5% bauxite, 87% tabular alumina, and 8% of a 75% phosphoric acid solution. The mix may be used immediately or dried at about 52.degree. C for future use with the addition to the thus heated mix, of about 1% water.
Cold setting bonds of the alkali silicate types are described in Rusoff et al U.S. Pat. No. 3,193,402 and Downs U.S. Pat. No. 2,429,872. The Downs patent discloses a mortar mix comprised of highly refractory grains some of which are coated with a highly aluminous glass, a cold setting bond such as sodium silicate, a small amount of an alkali fluoride, and a borate glass with a softening temperature of around 800.degree. C. Rusoff et al teaches a gunning cement composed of a dead burned refractory aggragate, an alkali silicate bonding agent, and particles of a pitch (e.g. coal tar pitch) which has a softening point high enough so that it does not soften prior to application of the cement mix and which decomposes under heat leaving a carbonaceous residue, and a clay type plasticizing component such as hectorite, bentonite, ball clay or the like.
Griggs et al U.S. Pat. No. 3,086,876 is a refractory patching cement that sets rapidly and exothermically thus developing substantial strength in a short period of time, avoiding lengthy furnace shut down periods. The patching compound is a mixture of aluminum, magnetite, a fluxing halide like sodium chloride or fluoride, and a refractory grain such as calcined dolomite. Silicon may be incorporated as an optional addition depending upon the furnacing operation involved. The mixture ignites and undergoes an exothermic reaction at about 1093.degree. C. to form a highly refractory and homogeneous patch.
Burrows in U.S. Pat. No. 3,793,040 discloses a refractory mix suitable for in-place-firing to produce furnace linings. The mix consists of a size graded refractory oxide grain, with up to 85% by volume of said grain having a coating of glass thereon. The glass is a high temperature softening glass, requiring a heat treatment of at least 1300.degree. C to facilitate coating of the glass on the refractory aggregate. The glass is present in an amount of from 2 to 15% by volume of the total composition.
All of the prior art cements suffer to a greater or lesser extent, from one or more disadvantges such as: the requirement of very high temperatures to set the cement; a period in its thermal history, prior to the initiation of high temperature ceramic bond formation, where the structure formed is mechanically weak; and, the lack of versatility which prevents the use of any single prior art cement as a dry vibration cement, ramming cement, and gunning cement.